The present invention relates to a method and apparatus for continuously monitoring the instantaneous movement of a surface whose motion is non-uniform and for measuring torsional vibrations in rotary machine parts.
If such vibrations are not sufficiently controlled they lead very quickly to fatigue failure of the rotary parts. There is, therefore, a need for apparatus capable of realiably detecting and measuring torsional vibrations in rotary machine parts. Existing methods for measuring such vibrations in rotating shafts involve the use of slip rings or slotted discs which are physically mounted on the shaft. Using slotted discs, a transducer monitors the number of slots which pass it in each revolution of the shaft and variations in the rate at which the slots reach the transducer over a period of time provide an indication of the level of torsional vibration in the shaft. As it is necessary to mount the slotted disc on the shaft each time a measurement is to be made, this method is time-consuming and cannot be used where access to the shaft is restricted and there is not sufficient room for the slotted disc to be mounted on the shaft. Furthermore, this method has a limited frequency response over the frequency ranges which are of interest.
One known device which utilizes a non-contact method and which overcomes some of these disadvantages is he laser-Doppler velocimeter. A beam from a laser source is split, in a conventional manner, to form two parallel beams which are focused by a lens so that they intersect at the surface of the rotating shaft. The frequency of the light which is backscattered at the surface of the shaft is Doppler shifted because the shaft surface is moving. As the two laser beams are coherent, the backscattered light from the two beams heterodynes at a detector positioned to receive it and the detected intensity is modulated at a frequency which is a function of the speed at which the surface is moving. Consequently, variations in the modulation frequency as the shaft revolves provide an indication of the level of torsional vibration.
Although this device avoids the need for mounting slip rings or slotted discs on the shaft, it has a number of disadvantages. The velocimeter must be arranged so that the shaft surface lies within the region where the beams intersect. As this region is generally only a fraction of a millimeter in length, accurate positioning of the device is necessary and hand-held use is not possible. Furthermore, this method is, of course restricted to shafts of circular cross-section. Again, in circumstances where access to the shaft is restricted, it may not be possible to position the device so that the intersection of the beams occurs at the shaft surface at all. It is also desirable that the modulation frequency should be within the frequency range for which the electronic circuitry used to process the detector output gives an optimum response. The modulation frequency is critically dependent on the speed of movement of the shaft surface and the velocimeter cannot, therefore, easily be used to measure a very wide range of speeds. The only other factors on which the modulation frequency is dependent are the wavelength of the laser light and the angle at which the beams intersect. Neither of these can easily be varied to adjust the modulation frequency. The cross-beam velocimeter is also susceptible to lateral movement of the shaft and provides an incorrect indication of the speed of movement of the shaft surface if the axial motion of the shaft has a velocity component in the plane defined by the incident laser beams.